An echo is simply a sound wave bouncing off a hard surface and returning to the listener.

The longer version

A hard flat wall reflects most of the energy that strikes it, sending the wave back the way it came. If the round trip is long enough, your ear separates the return from the original and hears a distinct repeat. Roughly a fifteenth of a second of delay is enough for the brain to register two events.

That delay implies the surface sits some distance away, since the wave must travel out and back. Short delays under about a twentieth of a second instead blend into the original and colour the tone rather than repeating it. Cliffs, canyon walls and empty halls all behave as mirrors for these waves.

Where this entry sits

Sound and Music gathers facts about how vibrations move through air and water, how ears turn them into meaning, and how instruments and notation shape what listeners hear. Expect echoes, pitch, volume, acoustics, and the odd silence in between. The full list sits on the Sound and Music page, where every entry is listed in order.